1. Field of the Invention
This invention relates to a method and a system implementation for determining the oxygen saturation (SO2) of blood in a blood vessel or body organ. The invention may employ invasive or non-invasive measurement techniques and is suitable for determining blood oxygen saturation in patients in any context, for example, central venous SO2 monitoring, pulmonary artery SO2 monitoring, extracorporeal SO2 monitoring, amputation level assessment, free-flap SO2 monitoring, etc.
2. Description of the Related Art
The standard way to measure blood oxygen saturation in a patient is to direct light into or through the blood, to measure the intensity of the light at either discrete wavelengths or over a substantially continuous spectral range after transmission through or reflection by the blood, and then to calculate SO2 as a function of the measured intensity values. Such devices are described, for example, in International Patent Application No WO94/03102.
Many factors reduce the accuracy of known SO2 monitors. Beginning with the light source itself, it must be able to produce light at a well-defined wavelength, or over a well-defined wavelength range, and it should do so stably over the life of the measurement instrument—there is no point measuring light absorption at a wavelength that is not produced with enough intensity to allow for a useful range of detection.
Getting the light to blood is also affected by various irregularities. When the light is directed into the blood using a non-invasive device such as a finger or ear lobe cuff, for example, inhomogeneities and irregularities in the body tissue between the light-generating device and the blood can influence light transmission in sometimes hard-to-estimate ways, which have nothing to do with the degree of blood oxygen saturation.
One irregularity that degrades the accuracy of most non-invasive monitors is patient motion, that is, motion artifact, which leads to a change in the path length of the light through the biological tissue and hence to a variation in the intensity of the detected transmitted or reflected light. This problem is in fact so great that it can render these devices inoperative for long periods of time. The problem is particularly severe in critical health care applications, were continuous monitoring is essential.
Generally, medical practitioners desire to measure arterial oxygen saturation (SaO2). Accordingly, most conventionally used pulse oximeters measure SaO2. The device described in WO 94/03102, for example, attempts to address the problem of motion artifact in measuring SaO2 by transmitting into the blood not only n predetermined wavelengths of light, but also an additional wavelength that makes it possible to cancel the motion artifact. Although WO 94/03102 broadly describes the use of a plurality of wavelengths (including the n+1 motion artifact wavelength) the device exemplified uses three wavelengths. However, in practice, the three wavelengths proposed in WO 94/03102 are not sufficient to overcome motion sensitivity.
Yet another factor that reduces the accuracy of non-invasive SO2 monitors is skin pigmentation: Many existing optical devices do not take into account the variations in transmitted light caused by with varying skin colors, which range from fair through brown to black as the concentration of melanin increases. The peak of melanin's absorption spectrum is at roughly 500 nm, decreasing almost linearly with increasing wavelength. Melanin is present in the epidermis; thus, in very high concentrations as is the case in black skin, it can mask the absorption of hemoglobin in the dermis. Even in brown skin, the absorption by melanin is superimposed on that of hemoglobin so that any algorithm which uses the shape of the absorption spectrum to produce an SO2 estimate needs to compensate for this fact.
International Patent Application No WO 00/09004 describes an optical device which is adapted to measure blood oxygen saturation. The device operates by passing light through biological tissue to monitor the transmitted or reflected output signal from a photodetector of this device continuously. However, one difficulty with the device of the prior art is the fact that the use of a limited number of wavelengths as in WO 00/09004 results in a poor signal-to-noise ratio in the detected signal. This reduces the accuracy of the SO2 determination. Further, this limited-wavelength technique is also more prone to ambient interference e.g. fluorescent lighting, etc.
One way to reduce the impact of the factors mentioned above is to measure SO2 invasively. In these applications, light is usually directed into blood by means of catheter-mounted or enclosed optical fibers. The light intensity measured to determine an absorption spectrum for the blood is then usually that of reflected rather than transmitted light. The obvious disadvantage of invasive monitors is the same as for any other invasive device: patient discomfort and the need for great care in positioning the sensor.
Regardless of whether the arrangement used to monitor SO2 is invasive or non-invasive, there is still the problem of converting the measured light spectrum—which comprises intensity values measured at several and sometimes very many wavelengths—into a single, accurate SO2 value, and to do so quickly enough to be useful in real-time, continuous patient monitoring. There is therefore a standing need to improve the accuracy and reliability of SO2 monitors.